As materials used for devices such as semiconductor device, photosensitive device for electrophotography, line sensor for image input, image pick-up device, photovoltatic device and other various electronic and optical devices, etc., deposition films of amorphous semiconductor such as amorphous silicon compensated, for example, with hydrogen atoms or halogen atoms (for example, fluorine, chlorine) (hereinafter referred to as "A-Si(H, X)") have been proposed and several of then have been put to practical use.
It has been known that such deposition films are formed by a plasma CVD method, that is, a method of decomposing starting material gas with DC current, high frequency wave, microwave or glow discharging and then forming a thin-film deposition layer on a substrate such as made of glass, quartz, heat resistant synthetic resin film, stainless steel or aluminum, as well as various devices therefor have been proposed.
Plasma CVD method of using glow discharging decomposition by microwave has particularly been noted industrially in recent years, and several kind of apparatus for practicing such microwave plasma CVD method have been proposed. They can be generally classified into the following types:
(1) A so-called ECR type plasma CVD apparatus in which plasmas are formed by electron cyclotron resonance (ECR) in a plasma generation chamber and the plasmas are introduced into a film-forming chamber. PA1 (2) A so-called direct introduction type microwave plasma CVD apparatus in which the microwave electric discharge power is directly introduced into a film-forming chamber thereby causing glow discharge plasmas.
FIG. 3 shows a direct introduction type microwave plasma CVD apparatus of the latter type developed by the present inventors. FIG. 3 is a schematic cross sectional view illustrating a typical embodiment of the aforementioned direct introduction type microwave plasma CVD apparatus.
In FIG. 3, there are shown a microwave introduction section 301, a vacuum chamber 302, a substrate (cylindrical) 303 (in a case where the substrate is a plate-like shape, deposition films are formed while bringing the plate-like substrates in close contact with the surface, for example, of an aluminum cylinder), a heater 304 for heating the substrate, an evacuating buffer plate 305, a vacuum seal mechanism 306, a cooling system introduction section 307, a motor 308 for rotating the substrate, a rotational shaft 309 for the substrate, substrate holders 310, 311, plasmas 312, a microwave introduction window 313 and a discharge space A respectively.
The apparatus shown in FIG. 3 has a pseudo circular cavity resonator structure in which a plurality of substrate 303, 303, --- are disposed in a circular arrangement within a vacuum chamber 302 to prepare a cylindrical space (discharge space A) at the central portion of the vacuum chamber and microwave power is charged from the cylindrical end face along at least one direction, thereby causing electric discharge. The microwave introduction section 301 can be designed relatively simply with no requirement for disposing large size solenoid coils or ECR cavity, etc. as in the case of the ECR (electron cyclotron resonance) type plasma CVD apparatus described above. In addition, by using the vacuum chamber 302 or the inner structure thereof as a cavity oscillator, it can supply a greater electric power than the ECR type plasma CVD apparatus and accordingly, it has a merit that the film-forming velocity is relatively high, the gas decomposing efficiency is about 100% and it is suitable to the mass production of the deposition films formed to the substrate of a great area.
FIG. 4 is a schematic cross sectional view schematically, showing in an enlarged scale, the introduction section of the microwave power in the apparatus shown in FIG. 3. In FIG. 4, the are shown a microwave introduction window 401 made of such material as capable of efficiently transmitting microwave power to the inside of the vacuum chamber and maintaining vacuum tightness, for example, formed with quartz glass, alumina ceramics, etc., a vacuum chamber wall 402, a vacuum seal 403 and a microwave guide tube 404. The waveguide tube guide 404 is connected by way of a matching device (not illustrated) and an isolator (not illustrated) to a microwave power source (not illustrated).
The deposition films are formed by using the apparatus as described below.
At first, a plurality of substrates 303, 303, --- are disposed within the vacuum chamber 302, the substrate 303 is rotated by the motor 308 for rotating the substrate and a diffusion pump (not illustrated) is actuated to reduce the pressure within the vacuum chamber to less than 10.sup.-6 Torr. Subsequently, the temperature of the substrate is controlled to a predetermined temperature from 50.degree. C. to 400.degree. C. by using the heater 304 for heating the substrate. When the substrate 303 is heated to a predetermined temperature, predetermined starting material gas, for example, silane gas (SiH.sub.4), hydrogen gas (H.sub.2), etc. in the case of forming A-Si(H, X) films into the discharge space A from the gas cylinder (not illustrated) to adjust the inner pressure in the discharge space A to a predetermined pressure of lower than 10.sup.m Torr. After the inner pressure has been stabilized, microwaves at a frequency of higher than 500 MHz, preferably , 2.45 GHz is generated by a microwave power source (not illustrated) to introduce the microwave energy by way of the microwave introduction section 301 to the discharge space A.
Starting material gas in the vacuum chamber is decomposed by the energy of the microwave and film deposition is caused on the substrate 303 to form a deposition film.
When the present inventors formed an A-Si(H, X) film by the apparatus shown in FIGS. 3 and 4 and using monosilane gas (SiH.sub.4) as the starting material gas, the gas decomposing efficiency was about 100% and the deposition velocity was about 100 .ANG./sec. The amount of the microwave power supplied in this case was 1 KW at the maximum in total. As apparent from the result, according to the microwave plasma CVD apparatus, a much greater deposition velocity can be obtained, which is about 10 times as high as the velocity in the case of using the conventional plasma CVD apparatus using high frequency wave power at a frequency of 13.56 MHz.
However, in the construction of the apparatus shown in FIGS. 3 and 4, the film is also deposited to the microwave introduction section 301, particularly, on the microwave introduction window 401 for supplying the microwave from the atmosphere to the vacuum chamber to worsen the propagation efficiency of the microwave to the inside of the vacuum chamber. Accordingly, it is difficult to supply the microwave into the vacuum chamber always at a stabilized state and, as a result, there is a problem that the control is difficult for the condition of the charged microwave power for efficiently forming the deposition film stationarily at high quality. Furthermore, if the thickness of the deposition film exceeds about 2 um, since the microwave transmission becomes remarkably difficult, it is required to replace the microwave introduction window 401 after successive formation of film, that is, several to 10 and several times. Although the microwave introduction window 401 is made exchangeable at present, there are problems that the time required for exchange upon attachment and detachment is innegligible, a great number of spare parts have to be arranged for exchange and, further, it needs additional step and cost for the cleaning works of removing films deposited to the detached window.